In order to operate modern internal combustion engines and to comply with strict emission limiting values, an engine controller uses what is referred to as the cylinder charge model to calculate the air mass enclosed in a cylinder per working cycle. According to the modeled air mass and the desired ratio between the quantity of air and quantity of fuel (Lambda), the corresponding quantity of fuel setpoint value (MFF_SP) is injected by means of an injection valve which is also referred to as an injector in this document. In this way, the quantity of fuel which is to be injected can be dimensioned in such a way that a value for Lambda which is optimum for the exhaust gas post-treatment in the catalytic converter is present. For direct-injection spark emission engines with internal mixture formation, the fuel is injected into the combustion chamber with a pressure in the range from 40 to 200 bar.
The main request made to the injection valve is, as well as the tightness with respect to an uncontrolled output fuel and the preparation of the jet of the fuel to be injected, precise metering of a predefined setpoint injection quantity.
In particular, in the case of supercharged, direct-injection spark emission engines a very large quantity spread of the required quantity of fuel is necessary. It is therefore necessary for a maximum quantity of fuel MFF_max per working cycle to be metered, for example, for the supercharged operating mode at the full load of the engine, whereas in the operating mode close to idling a minimum quantity of fuel MFF_min has to be metered. The two characteristic variables MFF_max and MFF_min define here the limits of the linear working range of the injection valve. This means that for these injection quantities there is a linear relationship between the electric actuation duration (Ti) and the injected quantity of fuel per working cycle (MFF).
For direct injection valves with a coil drive, the quantity spread, which is defined at a constant fuel pressure as the quotient between the maximum quantity of fuel MFF_max and the minimum quantity of fuel MFF_min, is approximately 15. For future engines with the emphasis on carbon dioxide reduction, the cubic capacity of the engines is made smaller and the rated power of the engine is maintained or even raised by means of corresponding engine supercharging mechanisms. As a result, the requirement which is made of the maximum quantity of fuel MFF_max corresponds at least to the requirements of an induction engine with a relatively large cubic capacity. The minimum quantity of fuel MFF_min is, however, determined by means of the operating mode which is close to idling and the minimum air mass in the overrun mode of the engine with a reduced cubic capacity, and said minimum quantity of fuel MFF_min is therefore made smaller. In addition, direct injection permits distribution of the entire fuel mass along a plurality of pulses which permits, for example, compliance with more stringent emission limiting values in a catalytic converter heating mode by means of what is referred to as mixture stratification and a later ignition time. For future engines, for the above-mentioned reasons there will be increased demands made of both the quantity spread and the minimum quantity of fuel MFF_min.
In known injection systems, in the case of injection quantities which are smaller than MFF_min, a significant deviation of the injection quantity from the nominal injection quantity occurs.
This symmetrically occurring deviation is mainly due to fabrication tolerances at the injector as well as to tolerances of the output stage which actuates the injector in the engine controller, and therefore to deviations from the nominal actuation current profile.
The electric actuation of a direct injection valve typically occurs by means of a current-controlled full-bridge output stage. Under the peripheral conditions of a vehicle application it is only possible to achieve a limited accuracy of the current profile which is applied to the injector. The resulting variation in the actuation current as well as the tolerances at the injector have significant effects on the achievable accuracy of the injection quantity, in particular in the region of MFF_min and below.
The characteristic curve of an injection valve defines the relationship between the injected quantity of fuel MFF and the duration Ti of the electric actuation as well as of the fuel pressure FUP (MFF=f(Ti, FUP)). The inversion of this relationship Ti=g (MFF_SP, FUP) is used in the engine controller to convert the setpoint quantity of fuel (MFF_SP) into the necessary injection time. The influencing variables which are additionally included in this calculation, such as for example the internal pressure of the cylinder during the injection process, the temperature of the fuel and possible variations of the supply voltage, are omitted here for the sake of simplification.
FIG. 1a shows the characteristic curve of a direct injection valve. In this context, the injected quantity of fuel MFF is plotted as a function of the duration Ti of the electric actuation. As is apparent from FIG. 1a, a working range which is linear to a very good approximation is obtained for durations Ti longer than Ti_min. This means that the injected quantity of fuel MFF is directly proportional to the duration Ti of the electric actuation. For durations Ti shorter than Ti_min, a strongly nonlinear behavior is obtained. In the illustrated example, Ti_min is approximately 0.5 ms.
The gradient of the characteristic curve in the linear working range corresponds to the static flow through the injection valve, i.e. the fuel through-flow rate which is achieved continuously in the case of complete valve stroke. The cause of the nonlinear behavior for durations Ti is shorter than approximately 0.5 ms or for quantities of fuel MFF<MFF_min is, in particular, the inertia of an injector spring mass system and the chronological behavior during the buildup and reduction of the magnetic field by a coil, which magnetic field actuates the valve needle of the injection valve. As a result of these dynamic effects, the complete valve stroke is no longer reached in what is referred to as the ballistic region. This means that the valve is closed again before the structurally predefined end position, which defines the maximum valve stroke, has been reached.
In order to ensure a defined and reproducible injection quantity, direct injection valves are usually operated in their linear working range. Currently, operation in the nonlinear range is not possible since owing to the above-mentioned tolerances in the current profile and mechanical tolerances of injection valves (for example prestressing force of the closing spring, stroke of the valve needle, internal friction in the armature/needle system), a significant systematic error occurs in the injection quantity. For a reliable operating mode of an injection valve, this results in a minimum quantity of fuel MFF_min per injection pulse, which minimum quantity of fuel MFF_min has to be at least provided in order to be able to implement the desired injection quantity accurately in terms of the quantity. In the example illustrated in FIG. 1a, this minimum quantity of fuel MFF_min is somewhat smaller than 5 mg.
FIG. 1b shows for the nonlinear operating range the respective deviation of the injection quantity relative to the nominal current profile (ΔI=0%) for relative errors in the current profile of varying severity.
The various relative errors in the current profile are −10%, −5%, −2.5%, +2.5%, +5% and +10% here. In the linear region which is not illustrated, and which starts at Ti=Ti_min=0.5 ms, an error in the current profile only has a weak effect on the accuracy of the quantity. However, starting from Ti<Ti_min and respectively MFF<MFF_min the quantity error increases significantly. Significant errors in the accuracy of the quantity occur in particular for injection times in the ballistic region.
The electric actuation of a direct injection valve which usually takes place by means of current-controlled full-bridge output stages of the engine controller. A full-bridge output stage makes it possible to supply the injection valve with a on-board power system voltage of the motor vehicle and alternatively with a boost voltage. The boost voltage (U_boost) can be, for example, approximately 60V. The boost voltage is usually made available by means of a DC/DC converter.
FIG. 2 shows a typical current actuation profile I (thick continuous line) for a direct injection valve with a coil drive. FIG. 2 also shows the corresponding voltage U (thin continuous line) which is applied to the direct injection valve. The actuation is divided into the following phases:
A) Pre-charge phase: during this phase of the duration t_pch, the bridge circuit of the output stage applies to the battery voltage U_bat, which corresponds to the on-board power system voltage of the motor vehicle, to the coil drive of the injection valve. When a current setpoint value I_pch is reached, the battery voltage U_bat is deactivated by a two-point controller and U_bat is switched on again after a further current threshold is undershot.
B) Boost phase: the pre-charge phase is adjoined by the boost phase. For this purpose, the output stage applies the boost voltage U_boost to the coil drive until a maximum current I_peak is reached. As a result of the rapid buildup of current, the injection valve opens in an accelerated fashion. After I_peak has been reached, a free-wheeling phase follows up until the expiry of t_1 and during said free-wheeling phase the battery voltage U_bat is in turn applied to the coil drive. The duration Ti of the electric actuation is measured from the start of the boost phase. This means that the transition into the free-wheeling phase is triggered by the predefined maximum current I_peak being reached. The duration t_1 of the boost phase is permanently predefined as a function of the fuel pressure.
C) Commutation phase: after the expiry of t_1, a commutation phase follows. Deactivation of the voltage results here in a self induction voltage which is limited substantially to the boost voltage U_boost. The voltage limitation during the self induction is composed of the sum of U_boost and of the forward voltages of a recovery diode and forward voltages of what is referred to as a free-wheeling diode. The sum of these voltages is referred to below as a recovery voltage. On the basis of a differential voltage measurement, on which FIG. 2 is based, the recovery voltage is formed in a negative fashion in the commutation phase.
As a result of the recovery voltage, a current flow is produced through the coil, which flow reduces the magnetic field. The commutation phase is timed and depends on the battery voltage U_bat and on the duration t_1 of the boost phase. The commutation phase ends after the expiry of a further duration t_2.
D) Holding phase: the commutation phase is adjoined by what is referred to as the holding phase. Here, the setpoint value for the holding current setpoint I_hold is controlled by means of the battery voltage U_bat, again by means of a two-point controller.
E) Deactivation phase: deactivation of the voltage results in a self induction voltage which, as explained above, is limited to the recovery voltage. This results in a current flow through the coil, which flow now decreases the magnetic field. After the recovery voltage which is formed here in a negative fashion has been exceeded, current does not flow anymore. This state is also referred to as “open coil”. Owing to the ohmic resistances of the magnetic material, the eddy currents which are induced during the reduction of the field of the coil decay. The reduction in the eddy currents leads in turn to a change in the field in the magnetic coil and therefore to voltage induction. This induction effect leads to the voltage value at the injector rising from the level of the recovery voltage to the value “zero” according to the profile of an exponential function. After the reduction of the magnetic force, the injector closes by means of the spring force and the hydraulic force which is caused by the fuel pressure.
The described actuation of an injection valve has the disadvantage that the precise time of closing of the injection valve or of the injector cannot be determined in the “open coil” phase. Since a variation of the injection quantity correlates to the resulting variation in the closing time, the absence of this information results, in particular in the case of very small injection quantities which are smaller than MFF_min, in considerable uncertainty regarding the quantity of fuel which is actually introduced into the combustion chamber of a motor vehicle engine.